1. Field of the Invention
This invention relates to process control systems, and more particularly to a proportional plus integral controller for use in such systems.
2. Description of the Prior Art
Proportional controllers are employed in process control systems to enable a controlled system variable to be maintained at a system set point by providing a feedback signal related in amplitude and polarity to deviations of the controlled variable from the set point.
In such systems, the controlled variable is monitored by a condition sensing device which provides a linearly changing command signal indicative of deviations of the controlled variable from the system set point. A process control apparatus is responsive to the command signal to effect an adjustment in the value of the manipulated variable to maintain the controlled variable at the system set point.
For example, in a heating or air conditioning control system, the controlled variable may be the temperature of a given enclosure or room and the manipulated variable may be hot air under pressure admitted to the enclosure or room. The condition sensing device may include a thermostat located in the room, which provides a position command to an actuator device. This position command is proportional to the difference between the room temperature and the thermostat set point temperature. The actuator device responsively controls the positioning of a damper to increase or decrease the rate at which hot air is admitted into the room or enclosure in accordance with the heating requirements of the room.
In known proportional controllers which provide only proportional action, the deviation of the controlled variable from the set point, or error, is never fully eliminated since it is necessary for an error to exist to command a system output other than the one neutral output at which the system was calibrated. This inherant deviation between the actual process condition, or controlled variable, and the desired process condition, or set point, is commonly referred to as "droop". In comfort control loops, a relatively small droop can be felt by human occupants of a temperature controlled space.
Droop may be lowered through the use of a high gain amplifier in the proportional control loop. However, system stability requirements place a limit on the highest attainable gain and thus, on the minimum droop. Droop is eliminated through the use of controllers which provide proportional plus automatic reset action, sometimes called integral action.
Although proportional plus integral controllers provide the desired elimination of droop, control loops employing an automatic reset control function are characterized by large overshoots in the event of process upsets as may be caused by start-up or large changes in set point, for example. In such instances, it may be desirable to provide only proportional action until the controlled variable approaches the set point and then to perform a bumpless transfer to the proportional plus integral mode. It is also desirable to switch to a proportional only mode to make an initial adjustment of the loop proportional gain in order to adjust the controller for near optimum response.
It is also desirable to add the proportional plus integral feature to existing control loops by simply cutting the lines between the controller sensor and the actuator and inserting the proportional plus integral package.
It is also desirable to retain the set point and proportional gain control in the proportional sensor controller.